The present invention relates to a cryogenic refrigeration method for use in connection with a cryogenic temperature swing adsorption process to purify a vapor stream and thereby to produce a purified vapor stream. More particularly, the present invention relates to such a refrigeration method in which a heat transfer stream is liquefied against the vaporization of a liquid stream to in turn produce the vapor stream to be purified and is then re-vaporized against the liquefaction of the purified vapor stream.
Cryogenic temperature swing adsorption processes are utilized to purify streams of a variety of gaseous mixtures. They are so called "temperature swing" in that adsorption occurs at low temperatures and desorption from an adsorbent is at least initiated by heating the adsorbent. A very important application of such a cryogenic temperature swing adsorption process is in the purification of argon produced from an argon column of a cryogenic air separation plant. The argon produced from such a plant contains unacceptable levels of oxygen and nitrogen for many applications, but particularly, for those involved in the electronics industry. In order to produce argon at the requisite purity level, a stream of argon vapor from the argon column is sent to art adsorbent bed for purification. The adsorbent bed contains stages of Zeolite molecular sieve materials that are selected to preferentially adsorb oxygen and nitrogen and thereby cleanse the argon stream of nitrogen and oxygen impurities.
In order to have some continuity in production, two beds of adsorbent are operated out of phase. One of the beds, an on-line bed, receives the argon vapor to produce a purified argon product as a vapor stream. At the same time, the other of the beds, an off-line bed, is regenerated. Regeneration of the off-line bed is initiated by purging the bed with heated nitrogen. The passage of nitrogen through the bed heats the bed to initiate desorption and to remove residual oxygen from the bed. Thereafter, the continued flow of the nitrogen, which is ordinarily at ambient atmospheric temperature, cools the bed. The bed is then purged with all or part of the purified argon product from the on-line bed to remove the nitrogen. A point is reached at which all the nitrogen is removed and the argon flowing into the bed is serving to cool the off-line bed down for future service as an on-line bed. As is evident from this discussion, the production of a purified product is discontinuous in that all or part of that the purified argon product is contaminated with nitrogen during the purging of the off-line bed and hence, cannot be taken as a product. Thereafter, as the bed cools, full production of the purified argon product resumes, but the purified argon product has been heated by the off-line bed into a superheated state.
As can be appreciated, the purified argon product must be liquefied if it is to be stored. However, if the argon product being used for regeneration purposes is also to be stored, then the cooling duty of the liquefier will change because during regeneration of the off-line bed, the purified argon product will either not be taken and hence, there will be no refrigeration requirement, or the purified argon product will be in a highly superheated state as compared with the slightly superheated state of the argon product being produced from the on-line bed. Conventionally, liquefaction of the argon product is effected through use of a nitrogen stream taken from the high pressure column. However, the refrigeration demand and hence, the demand for the nitrogen stream varies with the particular phase of the temperature swing adsorption process. This in turn creates a cyclic loading on the air separation unit which is disadvantageous from both design control standpoints.
As will be discussed, the present invention provides a cryogenic refrigeration method that incorporates the variable capacity liquefaction necessary to liquefy a purified product produced by a temperature swing adsorption process without substantially changing the demand for a cryogenic refrigerant.